Such arrangements are used in optics for the eye for determining the radius of curvature of the cornea. This is so, for example, because a precise knowledge of the corneal curvature is required for fitting a contact lens.
An apparatus for measuring the curvature of the cornea is disclosed, for example, in DE No. 26 41 004 C2. With this known apparatus, light points are generated on a carrier surface arranged in front of the patient's eye with these light points being virtually imaged on the cornea to be measured; also, a position-sensitive detector is provided for determining the position of the virtual light point corresponding to the particular light point. The disadvantage here is that the required imaging quality of the virtual light points is dependent on the local surface characteristic of the cornea to be measured and can be influenced in the measuring system in only a very limited manner.
Another known method for measuring the form of curved optically effective surfaces, such as exemplified by the cornea of the eye, is the so-called light-section method which, for example, can be carried out with slit lamps of known configuration. In this method, a narrow slit-shaped light beam is transmitted meridianly onto the surface to be measured with the section curve conjointly defined by this "light plane" and the surface being visible from the side as a consequence of stray reflections. The method directly provides the location curve of the meridian section.
When measuring the corneal configuration of the living eye and with the patient fixing the eye onto a mark localized in the axis of the illumination, one guides the light section centrally through the pupil of the eye so that the position of the sight axis with respect to the corneal surface is definitively determined. One can also determine the thickness of the cornea with this method because not only is the form of the outer surface of the region to be measured detectable, but also the form of the inner surface.
The disadvantage of this method is that for the interpolation of the entire surface, successive pluralities of meridian curves must be provided and that a satisfactory precision can only be reached if photographic pictures having sufficient imaging standards can be measured with a measuring microscope and a computed correction corresponding to the conditions for making the pictures is undertaken. An automation of the method with raster-electronic imaging detectors is opposed by their limited resolution since the state of the art requires that the entire meridian section be detected in one traversal.